Salt deposits were already favoured as possible locations for storing radioactive waste from research centres and nuclear power plants back in the nineteen-sixties. International experts from Germany and the US, among others, justified this with the excellent properties of salt formations:
- Rock salt deforms plastically when under pressure, this generally prevents the occurrence of interconnected fissures or cracks. If cracks do open, these are rapidly closed again.
- Thanks to its mechanical properties, it is possible to excavate large chambers in rock salt without any special support measures.
- Rock salt has a high specific thermal conductivity. In contrast to other rocks (e. g. granite or clay), radioactive waste producing heat can be better stored in rock salt because of its favourable influence on the dissipation of the heat of radioactive decay.
- Rock salt deposits have proved extremely stable over periods of millions of years despite the solubility of their evaporite minerals. Most of the deposits in Germany were formed in the Zechstein and are more than 240 million years old. In spite of the dramatic geological events in their surroundings such as, for example, the flooding of the Northern German Basin, the uplifting of the Alps and several Ice Ages, the salt dome interiors have had no contact with water bearing layers since their formation.
- Germany has a large number of rock salt deposits. The geological, technical and mining experience gathered in more than one century of salt mining in Germany can be used for geological disposal. No other type of rock has been researched as thoroughly as rock salt.
At the end of the nineteen-sixties, the former salt mine Asse near Wolfenbüttel, Lower Saxony, was converted to serve as a research mine in order to be able to gain practical experience on the evaluation of salt as a disposal medium. From 1967 to 1978, various procedures and techniques for storing low- and medium-level wastes were developed and tested on a large scale. Today, 124,500 containers (drums) with low-level waste and 1,290 containers with intermediate-level waste are emplaced at the Asse salt mine. Following the completion of the storage tests, the Asse was exclusively used as an underground laboratory. International experts also took part in the planning and execution of tests.
In order to demonstrate that geological disposal of high-level, heat-developing waste and irradiated fuel assemblies in salt is possible, numerous scientific tests were carried out (e. g. temperature and irradiation tests with a cobalt (Co-60) source and neutron back-scattering tests with a neutron source).
Furthermore, typical characteristics of salt were investigated and monitoring measures were effected, such as stability calculations, the geotechnical components of a repository, ground water measurements and analyses, radiological emission and immission measurements, deformation and stress measurements as well as geophysical measurements for monitoring the rock-mechanical situation. The predicted and achieved positive results were crucial for the decision to store radioactive waste in salt formations in Germany.
